Moving coil drive unit

A description of the moving coil drive unit, as used in most loudspeakers.

Perhaps the best place to start is a 200 mm drive unit intended for low and
mid frequency reproduction. This isn't the biggest drive unit available, so
why are larger drive units ever necessary? The answer is to achieve a higher
sound level. A 200 mm drive unit only pushes against so much air. Increase the
diameter to 300 mm or 375 mm and many more air molecules feel the impact. The
next question would be, why are 300 mm or 375 mm drive units not used more often,
when space is available? The answer to that is in the behavior of the diaphragm:

The diaphragm must not bend in operation otherwise it will produce distortion.
It is sometimes said that the diaphragm should operate as a 'rigid piston'.

The diaphragm could be flat and still produce sound. However, since the
motor is at the center and vibrations are transmitted to the edges, the diaphragm
needs to be stiff. The cone shape is the best compromise between stiffness
and large diameter.

High frequencies will tend to bend the diaphragm more than low frequencies.
It takes a certain time for movement of the coil to propagate to the edge
of the diaphragm. Fairly obviously, at high frequencies there isn't so much
time and at some frequency the diaphragm will start to deviate from the ideal
rigid piston.

200 mm is a good compromise. It will produce enough level at low frequency
for the average living room, and it will produce reasonably distortion-free
sound up to around 4 kHz or so. When the diaphragm bends, it is called break
up, due to the vibration 'breaking up' into a number of different
modes. Break up, in this context, doesn't mean severe distortion
or anything like that. In fact most low frequency drive units are operated well
into the break up region. It is up to the designer to ensure that the distortion
created doesn't sound too unpleasant. By the way, it is often thought that a
larger drive unit will operate down to lower frequencies. This isn't quite the
right way to look at it. Any size of drive unit will operate down to as low
a frequency as you like, but you need a big drive unit to shift large quantities
of air at low frequency. At high frequency, the drive unit vibrates backwards
and forwards rapidly, moving air on each vibration. At low frequencies there
are fewer opportunities to move air, therefore the area of the drive unit needs
to be greater to achieve the desired level.

The material of the diaphragm has a significant effect on its stiffness. Early
moving coil drive units used paper pulp diaphragms, which were not particularly
stiff. Modern drive units use plastic diaphragms, or pulp diaphragms that have
been doped to stiffen them adequately. Of course, the ultimate in stiffness
would be a metal diaphragm. Unfortunately, it would be heavy and the drive unit
would be less efficient. Carbon fiber diaphragms have also been used with some
success. (It is worth noting that in drive units used for electric guitars,
the diaphragm is designed to bend and distort. It is part of the sound of the
instrument and a distortion-free sound would not meet a guitarist's requirements).

Moving up the frequency range: as we have said, the diaphragm will bend and
produce distortion. Even if it didn't, there would still be the problem that
a large sound source will tend to focus sound over a narrow area, becoming narrower
as the frequency increases. In fact, this is the characteristic of direct radiator
loudspeakers: that their angle of coverage decreases as the frequency gets higher.
This is significant in PA, where a single loudspeaker has to cover a large number
of people. (It is perhaps counter-intuitive that a large sound source will focus
the sound, but it is certainly so. A good acoustics text will supply the explanation).

Because of these two factors, higher frequencies are handled by a smaller drive
unit. A smaller diaphragm is more rigid at higher frequencies, and because it
is smaller it spreads sound more widely. Often the diaphragm is dome shaped
rather than conical. This is part of the designer's art and isn't of direct
relevance to the sound engineer, as long as it sounds good.

It might be stating the obvious at this stage, but a low frequency drive unit
is commonly known as a woofer, and a high frequency drive unit as a tweeter.

In loudspeakers where a low frequency drive unit greater than 200 mm is used,
it will not be possible to use the woofer up to a sufficiently high frequency
to hand over directly to the tweeter. Therefore a mid frequency drive unit has
to be used (sometimes known as a squawker!). The function of dividing
the frequency band among the various drive units is handled by a crossover.

David Mellor has been creating music and recording in professional and home studios for more than 30 years. This website is all about learning how to improve and have more fun with music and recording. If you enjoy creating music and recording it, then you're definitely in the right place :-)